A known method for analyzing the sulfur crosslinked structure of a rubber vulcanizate crosslinked with a sulfur-containing compound such as sulfur vulcanizing agent is to selectively cleave crosslinks by a reagent such as LiAlH4 or propane-2-thiol, and calculate, from the amount of swelling before and after the cleavage, the crosslink densities [mol/cm3] of monosulfide bonds (R—S1—R), disulfide bonds (R—S2—R), and polysulfide bonds (R—Sn—R (n≧3)) in the rubber vulcanizate according to the Flory-Rehner equation (see, for example, Non Patent Literature 1).